What led the revealers of the Buddhist tantras to name their method Vajrayāna, or Vajra Vehicle? Here I would like to suggest, to the certain surprise of some people I know, that the reasons are to be found in polarity symbolism developed already in the sūtras of the Great Vehicle, the Mahāyāna. For example, the Eight Thousand Transcendent Insight Sūtra, often dated by scholars to the first century ce, repeats hundreds of times the phrase “transcendent insight and skillful method,” as the two things most necessary for progressing toward Enlightenment. Not only that, there are at least a dozen places in the same scripture where transcendent insight is called the Mother[1] of Buddhas and Bodhisattvas, since it is She who gives birth to them. Although we find in this text no corresponding statement that skillful means or method (the translation ‘creative stratagem’ has also been suggested) is the Father, this would certainly be a logical step to take. The Gaṇḍavyūha Sūtra (perhaps several centuries later than the Eight Thousand) does explicitly take that step when it says, “Oh noble son! Skillful method is the father of all Bodhisattvas. Transcendent insight is the mother.”[2]

via Tibeto-logic: The Vajra in the Sūtras.